A new series of heterometallic 3d–4f single molecule magnets (SMMs) of general formula [Cr III 2 Dy III 2 (OMe) 2 (RN{(CH 2) 2 OH} 2) 2 (acac) 4 (NO 3) 2 ] (R ¼ Me, Et, nBu) is reported, displaying slow relaxation of the magnetization and magnetic hysteresis with non-zero coercive fields. Dynamic magnetic susceptibility experiments show that the three complexes possess anisotropy barriers of 34, 37 and 41 K (24, 29 and 26 cm À1); of similar magnitude to their {Co III 2 Dy III 2 } counterparts. The replacement of the diamagnetic Co III for paramagnetic Cr III ions results in significantly longer relaxation times, as observed via M(H) hysteresis at low temperatures, absent for the Co III complexes. The present complexes are also compared to those of a similar Cr III –Dy III complex of formula [Cr III 2 Dy III 2 (OMe) 2 (O 2 CPh) 4 (mdea) 2 (NO 3) 2 ] (mdeaH 2 ¼ N-methyldiethanolamine), which displays SMM behaviour with a larger anisotropy barrier of 77 K ($54 cm À1) and even longer relaxation times. We show that the long relaxation times compared to the Co III analogues are due to the significant magnetic exchange interactions between the Cr III and Dy III ions, resulting in the suppression of quantum tunnelling of the magnetization (QTM) and leading to a multilevel relaxation barrier. The height of the relaxation barrier in these Cr III systems is then shown to be directly related to the strength of the exchange interactions between the Cr III and Dy III ions, showing a clear route towards enhancing the slow magnetic relaxation of coupled Cr III –Dy III systems.